Projection operation in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Projection in Relational Algebra

Which of the following best describes the projection operation in relational algebra?

A It selects specific columns from a table, removing duplicates by default.

B It selects specific rows from a table based on a condition.

C It combines two tables by matching rows with common attributes.

D It deletes rows from a table permanently.

💡 Hint

Think about which part of the table (rows or columns) projection focuses on.

Projection in relational algebra extracts specific columns (attributes) from a relation (table). It also removes duplicate rows in the result.

📋 Factual

intermediate

Result of Projection Operation

Given a table Students(Name, Age, Grade), what will be the result of the projection operation π_Name, Grade(Students)?

A A table with only the first row's Name and Grade.

B A table with columns Name, Age, and Grade, including all rows.

C A table with columns Age and Grade, including all rows.

D A table with columns Name and Grade, including all rows, duplicates removed.

💡 Hint

Projection keeps only the specified columns and removes duplicates.

The projection π_Name, Grade returns only the Name and Grade columns from the Students table, removing any duplicate rows.

🔍 Analysis

advanced

Effect of Projection on Duplicate Rows

Consider a table Employees(ID, Department, Salary) with multiple employees having the same Department and Salary. What happens when you perform the projection π_Department, Salary(Employees)?

A The result includes only the Department column with duplicates removed.

B The result includes unique combinations of Department and Salary, duplicates removed.

C The result includes all columns with duplicates removed.

D The result includes all rows with Department and Salary, duplicates kept.

💡 Hint

Remember that projection removes duplicate rows after selecting columns.

Projection selects the specified columns and removes duplicate rows, so only unique Department and Salary pairs remain.

❓ Comparison

advanced

Projection vs Selection

Which statement correctly compares projection and selection operations in relational algebra?

A Both projection and selection choose rows but differ in filtering criteria.

B Projection chooses rows; selection chooses columns based on conditions.

C Projection chooses columns; selection chooses rows based on conditions.

D Both projection and selection choose columns but differ in filtering criteria.

💡 Hint

Think about whether each operation filters rows or columns.

Projection extracts specific columns, while selection filters rows based on conditions.

❓ Reasoning

expert

Projection Operation Output Size

Given a relation R(A, B, C) with 100 rows, where column A has 50 unique values, column B has 80 unique values, and column C has 100 unique values, what is the maximum number of rows in the result of π_A, B(R)?

A 100

B 80

C 50

D 130

💡 Hint

Consider that projection removes duplicates but the combination of A and B may be unique for each row.

The maximum number of rows after projecting columns A and B is at most the total number of rows in R, which is 100. It cannot exceed the original row count.